Integrated circuit comprising an organic semiconductor, and method for the production of an integrated circuit

ABSTRACT

An embodiment of the invention provides an integrated circuit having an organic field effect transistor (OFET) with a dielectric layer. The dielectric layer is prepared from a polymer formulation comprising: about 100 parts of at least one crosslinkable base polymer, from about 10 to about 20 parts of at least one di- or tribenzyl alcohol compound as an electrophilic crosslinking component, from about 0.2 to about 10 parts of at least one photo acid generator, and at least one solvent. Another embodiment provides a semiconductor fabrication method. The method comprises applying the polymer formulation to a surface of a substrate, drying the polymer formulation, crosslinking the polymer formulation after drying, and baking the polymer formulation after crosslinking.

This application is a divisional of U.S. Ser. No. 11/362,960 filed onFeb. 27, 2006, now U.S. Pat. No. 7,582,896 which application is acontinuation of International Application No. PCT/DE2004/001903, filedAug. 24, 2004, which designated the United States and was not publishedin English, and which is based on German Application No. 103 40 608.5filed Aug. 29, 2003, of which all applications are incorporated hereinby reference.

TECHNICAL FIELD

The invention relates generally to integrated circuit manufacturing andmore particularly to organic field effect transistors (OFET).

BACKGROUND

Systems comprising integrated circuits based on organic semiconductors,in particular organic field effect transistors (OFET), constitute apromising technology in the mass application sector of economicalelectronics. A field effect transistor is considered to be organicparticularly if the semiconducting layer is produced from an organicmaterial.

Since it is possible to build up complex circuits using OFETs, there arenumerous potential applications. Thus, for example, the introduction ofRF-ID (RF-ID: radio frequency identification) systems based on thistechnology is considered as a potential replacement for the bar code,which is susceptible to faults and can be used only in direct visualcontact with the scanner.

In particular, circuits on flexible substrates, which can be produced inlarge quantities in roll-to-roll processes, are of interest here.

Owing to the thermal distortion of most suitable economical substrates(e.g. polyethylene terephthalate (PET), polyethylene naphthalate (PEN)),there is an upper temperature limit of 130-150° C. for the production ofsuch flexible substrates. Under certain preconditions, for example athermal pretreatment of the substrate, this temperature limit can beincreased to 200° C. but with the restriction that, although thedistortion of the substrate is reduced, it is not prevented.

A critical process step in the case of electronic components is thedeposition of the dielectric layer, in particular the gate dielectriclayer, of an OFET. The quality of the dielectrics in OFETs has to meetvery high requirements with regard to the thermal, chemical, mechanicaland electrical properties.

Silicon dioxide (SiO₂) is the currently most frequently used gatedielectric in OFETs, based on the wide availability in semiconductortechnology. Thus, transistor structures in which a doped silicon waferserves as the gate electrode, and thermal SiO₂ grown thereon forms thegate dielectric are described. This SiO₂ is produced at temperatures ofabout 800-1000° C. Other processes (e.g. CVD) for the deposition of SiO₂on various substrates likewise operate at temperatures above 400° C. Agroup at PennState University has developed a process (ion beamsputtering) which makes it possible to deposit a high-quality SiO₂ atprocess temperatures of 80° C. This is described in the articles by C.D. Sheraw, J. A. Nichols, D. J. Gunlach, J. R. Huang, C. C. Kuo, H.Klauk, T. N. Jackson, M. G. Kane, J. Campi, F. P. Cuomo and B. K.Greening, Techn. Dig.-lot. Electron Devices Meet., 619 (2000), and C. D.Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson, M. G. Kane,I. G. Hili, M. S. Hammond, J. Campi, B. K. Greening, J. Francl and J.West, Appl. Phys. Lett. 80, 1088 (2002).

However, the high process costs and the low throughput aredisadvantageous here for mass-produced products.

It is also known that inorganic nitrides, such as, for example,SiN_(x′), TaN_(x), can be used. Similarly to the preparation ofinorganic oxides, the deposits of inorganic nitrides require hightemperatures or high process costs. This is described, for example, inthe article by B. K. Crone, A. Dodabalapur, R. Sarpeshkar, R. W. Filas,Y. Y. Lin, Z. Bao, J. H. O'Neill, W. Li and H. E. Katz, J. Appl. Phys.89, 5125 (2001).

It is also known that hybrid solutions (spin on glass) can be used.Organic siloxanes which can be prepared from a solution and can beconverted into “glass-like” layers by thermal conversion were described.The conversion into SiO₂ is effected either at high temperatures (about400° C.) or takes place only partly, which results in a reducedtransistor quality (in this context, cf. the article by Z. Bao, V. Kuck,J. A. Rogers and M. A. Paczkowski, Adv. Funct. Mater., 12, 526 (2002).

In addition, organic polymers, such as, for example, poly-4-vinylphenol(PVP), poly-4-vinylphenol-co-2-hydroxyethyl methacrylate or polyimide(PI), have already been used. These polymers are distinguished by theircomparatively simple processability. Thus, they can be used, forexample, from solution for spin coating or printing. The outstandingdielectric properties of such materials have already been demonstrated(cf. article by H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W.Radlik and W. Weber, J. Appl. Phys., vol. 92, no. 9, 2002, p.5259-5263).

It has also already been possible to demonstrate applications ininterconnect layers (ICs), the required chemical and mechanicalstabilities of the dielectric layers for the structuring thereof and thestructuring of the subsequent source-drain layer having been achieved bycrosslinking of the polymers (cf. article by M. Halik, H. Klauk, U.Zschieschang, T. Kriem, G. Schmid and W. Radlik, Appl. Phys. Lett., 81,289 (2002)). However, this crosslinking is effected at temperatures of200° C., which is problematic for the production of flexible substrateshaving a large area.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an integrated circuitcomprising an organic semiconductor and a method in which the productionof dielectric layers of an OFET is possible at low temperatures.

An embodiment of the invention provides an integrated circuit comprisingan organic semiconductor, preferably an organic field effect transistor(OFET). The OFET has dielectric layer that can be produced from apreferred polymer formulation comprising:

a) about 100 parts of at least one crosslinkable base polymer,

b) from about 10 to about 20 parts of at least one di- or tribenzylalcohol compound as the electrophilic crosslinking component,

c) from about 0.2 to about 10 parts of at least one photo acidgenerator, and

d) at least one solvent.

The integrated circuits according to the invention are in particularOFETs having organic layers, which have outstanding dielectricproperties. Owing to the specific polymer formulation used, theintegrated circuits can be produced in a simple manner at lowtemperatures (up to 150° C.). This polymer formulation can also be usedin principle in combination with other electronic components.

Preferably, at least one base polymer is a phenol-containing polymer orcopolymer, in particular poly-4-vinylphenol,poly-4-vinylphenol-co-2-hydroxyethyl methacrylate orpoly-4-vinylphenol-co-methyl methacrylate.

Preferably, at least one di- or tribenzyl alcohol compound aselectrophilic crosslinking component is 4-hydroxymethylbenzyl alcohol.

In preferred embodiments of the invention, at least one crosslinkingcomponent has one of the following structures:

wherein R₁ may comprise: —O—, —S—, —SO₂—, —S_(x)—, —(CH₂)_(x)—, x=1-10,and additionally:

wherein each occurrence of R₂ may independently comprise an alkyl having1 to 10 carbon atoms or an aryl.

The photo acid generator used may comprise at least one compound which,on exposure to UV light, generates a photo acid for transferring aproton to the hydroxyl group of a benzyl alcohol, in particular asulfonium or an iodonium salt.

Solvents may comprise an alcohol, in particular n-butanol, propyleneglycol monomethyl ethyl acetate (PGMEA), dioxane, N-methylpyrrolidone(NMP), γ-butyrolactone, xylene or a mixture.

For good processability, it is preferred that the proportion of basepolymer, crosslinking component and photo acid generator is a proportionbetween 5 and 20% by mass.

Other embodiments of the invention provide a method for producing anintegrated circuit, preferably one having an OFET. An OFET dielectriclayer may be prepared using a preferred polymer formulation. The polymerformulation is applied to a substrate, which may have a prestructuredgate electrode. Embodiments may further include carrying out aphoto-induced crosslinking reaction, thereby forming the gate dielectriclayer.

The photoinduced crosslinking reaction is preferably initiated byexposure to UV radiation. It is particularly preferred if, after theexposure, a heating step, in particular a post exposure bake, iseffected. Preferably, the temperature in the heating step is not morethan 140° C., preferably about 100° C.

The polymer formulation is preferably applied using spin coating,printing, or spraying. The crosslinking reaction may be effected underan inert gas atmosphere, preferably an N₂ atmosphere. Embodiments mayfurther include drying at about 100° C. after applying the polymerformulation and forming the polymer film. Forming the OFET may furtherinclude applying a source-drain layer to the gate dielectric layer.

Preferably, an active layer, which may include semiconducting pentacene,is applied to the source-drain layer. A passivating layer may bearranged on the active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below for a plurality ofembodiments with reference to the figures of the drawings.

FIG. 1 shows a schematic diagram of an organic field effect transistor;

FIG. 2 shows an example of a photoinduced crosslinking reaction of apolymeric gate dielectric comprising PVP and 4-hydroxymethylbenzylalcohol as a crosslinking agent;

FIG. 3 a shows a family of output characteristics of an OFET comprisingan electrophilically crosslinked gate dielectric;

FIG. 3 b shows a family of transmission characteristics of an OFETcomprising an electrophilically crosslinked gate dielectric; and

FIG. 4 shows a trace of an oscilloscope image OFETs are electroniccomponents which consist of a plurality of layers, all of which havebeen structured, in order to generate integrated circuits by connectionsof individual layers.

The following list of reference symbols can be used in conjunction withthe figures:

-   -   1 Substrate    -   2 Gate electrode    -   3 Gate dielectric layer    -   4 a Drain layer    -   4 b Source layer    -   5 Active layer    -   6 Passivating layer    -   7 Interconnect layer

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows the fundamental structure of such a transistor in a bottomcontact architecture.

A gate electrode 2, which is covered by a gate dielectric layer 3, isarranged on a substrate 1. As will be explained later, in an embodimentof the process according to the invention the substrate 1 with the gateelectrode 2 already arranged thereon constitutes the starting materialon which the gate dielectric layer 3 is applied. A drain layer 4 a and asource layer 4 b, both of which are connected to the activesemiconducting layer 5, are arranged on the gate dielectric layer 3. Apassivating layer 6 is arranged above the active layer 5.

The source layer 4 b produces a connection to the interconnect layer 7.

The deposition and processing of the gate dielectric layer 3 arepreferred for that embodiment of the invention described here.

The circuits according to the invention and the production thereof solvethe problem of the provision of OFETs having gate dielectric layers, inparticular with organic ICs having outstanding mechanical, chemical andelectrical properties in combination with low process temperatures.

An OFET has a dielectric layer that can be produced from a mixture(polymer formulation) comprising four components: a base polymer, acrosslinking component, a photo acid generator, and a solvent. Anembodiment of the circuit according to the invention which is mentionedhere by way of example has a polymer formulation comprising thefollowing components:

a) PVP as the crosslinkable base polymer,

b) 4-hydroxymethylbenzyl alcohol as an electrophilic crosslinkingcomponent,

c) triphenylsulfonium hexaflate as the photo acid generator (PAG),

d) e.g. alcohols, PGMEA as the solvent.

This polymer formulation is applied to a correspondingly preparedsubstrate 1 (gate structures 2 have already been defined on thesubstrate 1). The polymer formulation can be applied, for example, byprinting, spin coating or spray coating. By subsequent drying atmoderate temperatures (about 100° C.), the polymer formulation is fixedon the substrate.

Thereafter, an exposure step using UV light is effected, the wavelengthand the duration of the UV irradiation being dependent on the photo acidgenerator used. A photo acid, which initiates a crosslinking reaction ina subsequent heating step (preferably not more than 140° C., postexposure bake (PEB)), is generated from the photo acid generator.

FIG. 2 shows the photoinduced electrophilic crosslinking reaction of apolymeric gate dielectric for PVP by way of example with4-hydroxymethylbenzyl alcohol as the crosslinking component. The photoacid generator used is triphenylsulfonium hexaflate.

As a result of the photochemically induced crosslinking reaction,solubility differences are produced between crosslinked anduncrosslinked material. By using masks, a definition of exposed andunexposed parts is possible thereby, which can be used for structuringthe gate dielectric layer 3.

The use of the preferred process reduces the required crosslinkingtemperature by more than 60° C. compared with the methods known to date(cf. article by Halik et al. (2002)). The temperatures used are notcritical for the substrate.

The base polymer determines the fundamental properties of the gatedielectric layer 3. Suitable base polymers are in principle allphenol-containing polymers and copolymers thereof, such as, for example,poly-4-vinylphenol, poly-4-vinylphenol-co-2-hydroxyethyl methacrylate orpoly-4-vinylphenol-co-methyl methacrylate.

By the choice of the crosslinking component and the concentrationthereof in the polymer formulation, the mechanical properties of thepolymer layer and the resistance to chemicals can be decisivelycontrolled.

By the choice of the photo acid generator, wavelength and exposure dosefor the initiation of the crosslinking reaction can be controlled. Thetemperature of the post exposure bake (PEB) determines the duration ofthe crosslinking step since this is determined substantially by thediffusion of the photo generated acid.

The choice of the solvent determines the film formation properties ofthe formulation.

Two polymer formulations, which differ only in the proportion of thecrosslinking agent, are described below as examples.

Formulation 1 is a 10% strength solution in propylene glycol monomethylether acetate (PGMEA). 100 parts of base polymer, 10 parts ofcrosslinking agent and 2.5 parts of photo acid generator are present.

A mixture of 2 g of PVP (MW about 20,000) as base polymer and 200 mg of4-hydroxymethylbenzyl alcohol as crosslinking agent are dissolved in20.5 g of PGMEA as solvent on a shaking apparatus (about 3 hours).

Thereafter, 50 mg of triphenylsulfonium hexaflate as a photo acidgenerator are added and the total solution is shaken for a further hour.Before use, the polymer solution is filtered through a 0.2 μm filter.

Formulation 2 is a 10% strength solution in PGMEA. 100 parts of basepolymer, 20 parts of crosslinking agent and 2.5 parts of photo acidgenerator are present. The proportion of crosslinking agent is thereforetwice as high as in the formulation 1.

A mixture of 2 g of PVP (MW about 20,000) as base polymer and 400 mg of4-hydroxymethylbenzyl alcohol as crosslinking agent are dissolved in20.5 g of PGMEA as solvent on a shaking apparatus (about 3 hours).Thereafter, 50 mg of triphenylsulfonium hexaflate as a photo acidgenerator are added and the total solution is shaken for a further hour.Before use, the polymer solution is filtered through a 0.2 μm filter.

Film preparation: 2 ml of the formulation 1 were applied by means of aspin coater at 4000 rpm for 22 s to a prepared substrate (PEN(polyethylene naphthalate) having Ti gate structures). Thereafter,drying is effected at 100° C. for 2 min on a hotplate. The layer is thenexposed (wavelength 365 nm, duration 30 seconds), intensity ofirradiation 7 mW/cm²).

A post exposure bake is then effected at 140° C. in an oven under a 400mbar N₂ atmosphere for 20 minutes.

The film preparation for formulation 2 is effected analogously.

Structuring of the Gate Dielectric Layer:

The structuring is effected as stated in the examples, except that thecrosslinked polymer layer (gate dielectric layer 3) is exposed using abright field chromium mask (chrome on glass COG). After the postexposure bake step, uncrosslinked dielectric (i.e. parts of thedielectric layer 3 which were not exposed to light) is dissolved awaywith acetone. The structured dielectric layer 3 remains in the exposedparts.

The source-drain layer 4 a, 4 b is then deposited and structured bystandard methods (30 nm Au applied thermally by vapor deposition,photolithographic structuring and wet chemical etching with I₂/KIsolution).

The layer thickness of the gate dielectric layers 2 is 200 nm forformulation 1. The roughness of the layer is 0.7 nm on 50 μm.

The layer thickness of the gate dielectric layers is 210 nm forformulation 2. The roughness of the layer is 0.7 nm on 50 μm.

The transistors or circuits are completed by applying the activecomponent 5 (in this case pentacene) thermally by vapor deposition.Except for the passivating layer 6, the structure of an OFET accordingto FIG. 1 is thus produced.

Here, embodiments for a polymer formulation and the use thereof for theproduction of gate dielectric layers 3 at low temperatures for use inintegrated circuits based on OFETs are described. These gate dielectriclayers 3 are distinguished by outstanding thermal, chemical, mechanicaland electrical properties in addition to the low process temperature forthe production thereof.

FIG. 3 a shows a family of output characteristics of a pentacene OFETcomprising an electrophilically crosslinked gate dielectric. FIG. 3 bshows, for the same structure, the transmission characteristics of anOFET (μ=0.8 cm²/Vs, on/off ratio=10⁵). In FIG. 4, a trace of anoscilloscope diagram is reproduced. The characteristic of a 5-stage ringoscillator is shown, the ring oscillator operating with a signal lag of80 μsec per stage.

In connection with the figures, an OFET was used as the integratedcircuit in an embodiment. In principle, the integrated circuit accordingto the invention can, however, also have a different structure in whicha dielectric layer is used.

The invention is not limited in its execution to the above-mentionedpreferred embodiments. Rather, a number of variants which make use ofthe apparatus according to the invention and the method according to theinvention also in versions of fundamentally different types isconceivable.

1. A structural building block comprising a substrate with a layer,wherein the layer comprises a polymer formulation, the polymerformulation comprising: about 100 parts by mass of at least onecrosslinkable base; polymer selected from the group consisting ofpoly-4-vinylphenol, poly-4-vinylphenol-co-2-hydroxyethylmethacrylate andpoly-4-vinylphenol-co-methylmethacrylate; from about 10 to about 20parts by mass of 4 hydroxymethyl-benzyl alcohol as an electrophiliccrosslinking component; and from about 0.2 to about 10 parts by mass oftriphenysulfoniumhexaflat as a photo acid generator, whereby, upon aphotoinduced crosslinking reaction, the layer forms a gate dielectriclayer of an organic field effect transistor (OFET).
 2. The structuralbuilding block of claim 1, wherein the photo acid generator generates aphoto acid for transferring a proton to a hydroxyl group of the benzylalcohol on exposure to UV light.
 3. The structural building block ofclaim 1, further comprising a solvent comprising a material selectedfrom the group consisting of: n-butanol, propylene glycol monomethylether acetate (PGMEA), dioxane, N-methylpyrolidone (NMP),γ-butyrolactone, xylene, and combinations thereof.
 4. The structuralbuilding block of claim 1, wherein the crosslinkable base polymer, theelectrophilic crosslinking component, and the photo acid generatorcomprise about 5% to about 20% by mass of the polymer formulation.
 5. Anorganic transistor comprising: a gate structure disposed on a substrate;and a dielectric layer disposed over and around the gate structure, thedielectric layer comprising a polymer material, the polymer materialcomprising about 100 parts by mass of at least one crosslinkable basepolymer selected from the group consisting of poly-4-vinylphenol,poly-4-vinylphenol-co-2-hydroxyethylmethacrylate andpoly-4-vinylphenol-co-methylmethacrylate, from about 10 to 20 parts bymass of 4-hydroxymethyl-benzyl alcohol as an electrophilic crosslinkingcomponent, and from about 0.2 to about 10 parts by mass oftriphenysulfoniumhexaflat as a photo acid generator, wherein the basepolymer is crosslinked with the electrophilic crosslinking component. 6.The organic transistor of claim 5, wherein the gate structure comprisesTi.
 7. The organic transistor of claim 5, wherein the substratecomprises polyethylene naphthalate.
 8. The organic transistor of claim5, further comprising: a source layer disposed over a first portion ofthe dielectric layer; a drain layer disposed over a second portion ofthe dielectric layer, wherein the first and second portions of thedielectric layer are not electrically connected; and an active layerdisposed between the source and drain layers, the active layer disposedon the dielectric layer.
 9. The organic transistor of claim 8, whereinthe source and drain layers comprise Au.
 10. The organic transistor ofclaim 8, wherein the active layer comprises pentacene.
 11. An organictransistor comprising: a gate structure disposed on a substrate; adielectric layer disposed over and around the gate structure, thedielectric layer comprising a polymer formulation, wherein the polymerformulation comprises about 100 parts by mass of at least onecrosslinkable base polymer selected from the group consisting ofpoly-4-vinylphenol, poly-4-vinylphenol-co-2-hydroxyethylmethacrylate andpoly-4-vinylphenol-co-methylmethacrylate, from about 10 to 20 parts bymass of 4-hydroxymethyl-benzyl alcohol as an electrophilic crosslinkingcomponent, and from about 0.2 to about 10 parts by mass oftriphenysulfoniumhexaflat as a photo acid generator; a source layerdisposed over a first portion of the dielectric layer; a drain layerdisposed over a second portion of the dielectric layer, wherein thefirst and second portions of the dielectric layer are not electricallyconnected; and an active layer disposed between the source and drainlayers, the active layer disposed on the dielectric layer.
 12. Theorganic transistor of claim 11, wherein the active layer comprisespentacene, wherein the substrate comprises polyethylene naphthalate,wherein the gate structure comprises Ti, and wherein the source anddrain layers comprise Au.